1. Field of the Invention
The present invention relates to a semiconductor device of an insertion-mount-type and a semiconductor assembly module, more specifically relates to a semiconductor device capable of improving the solderability of leads, particularly improving the solderability of the leads for lead-free solder mounting of a power module and a semiconductor assembly module using the semiconductor device.
2. Description of the Related Art
In general, a semiconductor device of an insertion-mount-type is mounted on an external substrate by inserting leads protruding from the semiconductor device into through-holes of the external substrate. A structure of a conventional semiconductor device of an insertion-mount-type and a method for mounting the semiconductor device on an external substrate are described below by using the DIPIPM shown in FIGS. 23A–23D and FIG. 24 as an example.
As shown in FIGS. 23A–23D and FIG. 24, in the conventional semiconductor device 101, a power semiconductor element 102 and a control semiconductor element 103 are mounted on a copper lead frame 105 provided with leads 104. Both of the semiconductor elements 102 and 103 are electrically connected to their corresponding leads 104 by metallic thin wires 106 and 107. Moreover, the semiconductor device 101 is provided with a heat sink 108 for improving the heat release property. In this case, the lead frame 105 also serves as a circuit substrate. These members 102 to 108 are sealed by a plastic package 110.
As shown in FIGS. 23B and 23C, in the conventional semiconductor device 101 (DIPIPM), each of the leads 104 is constituted by a narrow front-end-side portion B1 to be inserted into the through-hole of an external substrate (not illustrated) and a proximal-end-side portion B2 wider than the portion B1. Moreover, to mount the semiconductor device 101 on the external substrate, the semiconductor device 101 and external substrate are aligned in the height direction so that the gap (interval) between them is kept constant.
The soldering of a semiconductor device of an insertion-mount-type to an external substrate generally uses the flow soldering method or point flow soldering method. According to these methods, soldering is performed by supplying flux to the leads and external substrate which are to be soldered and then preheating them, supplying melted solder, and thereby raising the temperature of the leads and external substrate up to the liquidus-line temperature of solder or higher.
Thus, the conventional semiconductor device 101 is also soldered to the external substrate by one of the above methods. In this case, a comparatively large amount of heat is discharged to the plastic package 110 through the portion B2 that is not inserted into the external substrate from the portion B1 of the lead 104 that is soldered. Therefore, because the temperature of the portion B1 of the lead 104 which should be soldered does not reach the liquidus-line temperature of solder under the soldering process, a problem occurs that imperfect soldering is made.
Moreover, when using Pb-free solder based on Sn whose consumption is increased in recent years from the viewpoint of environmental problems, the liquidus-line temperature rises by approx. 40° K. compared to the case of conventionally-used Sn—Pb eutectic solder. Therefore, the above problem is further actualized. This is because it is difficult to raise a process temperature such as a preheating temperature or melted-solder temperature in order to compensate the rise value (40° K.) of the liquidus-line temperature of solder.
In the case of a surface-mount-type semiconductor device, the soldering process generally uses a method of heating the whole structure to be soldered, that is, a method of heating a semiconductor device and a substrate, for example, a method such as flow soldering. Therefore, imperfect soldering due to heat dissipation from a lead to a resin package does not occur and thus, there is no problem.
For the above problem, as shown in FIG. 25, Japanese Laid-Open Patent Publication No. 76357/1988 discloses a method of making positions of height-adjusting lead-step portions 210, 211, and 212 different from each other in accordance with volumes of lead-embedding portions 204, 206, and 208 located in a resin package 201 in a semiconductor device in which a semiconductor chip 202, a metallic thin wire 203, and three types of leads 205, 207, and 209 are embedded in the resin package 201.
However, the above method has a problem that the solderability is not improved at all in the case of the lead 207 whose lead step portion 211 is closest to a substrate. To improve the solderability of a semiconductor device, it is clear that it is necessary to improve the solderability of every lead. In this point, it is insufficient to solve the problem by the above method. Particularly, in the case of solder mount using lead-free solder having a high liquidus-line temperature, there is a problem that imperfect soldering may frequency occur.
Moreover, to keep the gap between the semiconductor device and the substrate constant while contacting with the substrate, it is necessary to form two or more lead-step portions on the same-height places. However, in the case of this method, the position of the lead-step portion depends on the volume of a metallic lead in the lead-embedding portion. Therefore, to form two or more lead-step portions on same-height places, there is a problem that the design of the leads must be greatly restricted. However, when forming the lead-step portions on the same-height places between the metallic leads having volumes different from each other at the lead-embedding portion, there is a problem that preferable solderability may not be secured. Moreover, there is a problem that it is very complicated to decide the position of the lead-step portion by the above method.
Then, to avoid the above problem in accordance with an approach of raising the preheating temperature, the temperature of the whole external substrate is raised. Moreover, not only the semiconductor device 101 but also various electronic components are mounted on the external substrate and a considerable number of components having a low heat-resistant temperature (including an electrolytic capacitor) are present in the components. Therefore, the problem solution according to the approach of raising the preheating temperature is limited. Moreover, when the semiconductor device 101 or external substrate has an excessively high temperature due to preheating, flux looses an active force. Therefore, the semiconductor device 101 or external substrate cannot show its effect under the important soldering process and on the contrary, a problem occurs that the solderability is deteriorated.
Moreover, though an approach of avoiding the above problem by raising the temperature of melted solder to be supplied is made, the problem solution by the approach is limited from the viewpoint of the heat-resistant temperature of the component and deactivation of flux. Furthermore, an approach of increasing the process time is possible. In this case, however, the above problem is not solved because the temperature of the whole external substrate rises and moreover, a problem occurs that increase of the process time causes the cost of the semiconductor device to increase.
As described above, when the semiconductor device by using Pb-free solder is mounted, the liquidus-line temperature of the solder rises by 40° K. Therefore, the above problem becomes more remarkable and it is needless to say that solution by each of the above approaches becomes more difficult.